The present invention relates to a three-group zoom lens for a camera.
Conventionally, a three-group zoom lens has been widely used as a zoom lens for various cameras. Examples of such a three-group zoom lens are disclosed in Japanese Patent Provisional Publications Nos. HEI 11-52256 and HEI 10-39214. In each of the three-group zoom lenses disclosed in the publications, the first lens group consists of, from an object side, a negative lens, a negative lens and a positive lens. With this configuration, a diameter of the lens can be decreased and therefore the entire lens system can be downsized. However, in this type of zoom lens, distortion cannot be compensated sufficiently.
In Japanese Patent Provisional Publication No. HEI 10-293253, another configuration of three-group zoom lens is disclosed. In this three-group zoom lens, a first lens group consists of a positive lens, a negative lens, a negative lens and a positive lens. With this structure, since a positive lens is provided as a first lens of the first lens group, the distortion can be well compensated. In this zoom lens, the third lens group is moved for focusing. In this zoom lens, when a distance to be focused is relatively small, curvature of field and lateral chromatic aberration cannot be compensated sufficiently, and therefore, the optical performance of the zoom lens is deteriorated when an object is located at a relatively close position with respect to the camera.
It is therefore an object of the invention to provide an improved three-group zoom lens, which is a rear-focusing type three-group zoom lens, having an excellent optical performance over an range from a close object distance to infinity.
For the above object, according to the invention, there is provided a three-group zoom lens having, from an object side, a first group lens, a second group lens and a third group lens, the first lens group having a negative power, the second lens group having a positive power, the second lens group including an aperture stop, the third lens group consisting of a single positive lens. With this structure, the zoom lens is further configured such that, when a distance to be focused is infinity, only the first lens group and the second lens group move for zooming, and at least one positive lens included in the zoom lens has an aspherical surface whose positive power is greater at a portion farther from an optical axis thereof. Additionally, the zoom lens is configured to satisfy following conditions:
0.85 less than f2/f1 less than xe2x88x920.65;
and
5.0 less than f3/fw less than 8.0,
where, fw represents a focal length when the zoom lens is positioned at a wide-extremity, f1 represents a focal length of the first lens group, f2 represents a focal length of the second lens group, and f3 represents a focal length of the third lens group.
Optionally, the first lens group includes, from the object side, a positive lens, a negative meniscus lens, a negative lens and a positive lens, and wherein the first lens group satisfies a following condition:
0.37 less than f1n/f1 less than 0.60
where, f1n represents a combined focal length of negative lenses included in the first lens group.
Optionally or alternatively, the second lens group may include, from the object side, a positive lens, a cemented lens including a positive lens and a negative lens, and a positive lens, and the second lens group may satisfy a condition:
0.35 less than R/f2 less than 0.55
where, R represents a radius of curvature of an object side surface of the cemented lens.
Optionally or alternatively, the zoom lens is configured to satisfy a condition:
D2(w) less than D3(t)
where, D2(w) represents a distance between an image side surface of the second lens group and an image plane when the zoom lens is positioned at a wide-extremity, and wherein D3(t) represents a distance between a first surface of the third lens group and the image plane when the zoom lens is positioned at the wide-extremity and a distance to be focused is a closest focusable distance.
Optionally or alternatively, the zoom lens may satisfy following conditions:
0.0 less than (R2+R1)/(R2xe2x88x92R1) less than 1.3;
and
0.05 less than xcex94III(asp) less than 0.12,
where, R1 is a radius of curvature of an object side surface of the third lens group, R2 is a radius of curvature of an image side surface of the third lens group, and xcex94III(asp) is an aberration coefficient of astigmatic difference normalized such that the focal length of the zoom lens positioned at a tele-extremity is represented by one.